Fix Issue #1216 (ValueError with Amazon RDS)

Fix typo where wrong variable was used, causing ValueError.

cloud/amazon/rds_param_group.py

@@ -196,7 +196,7 @@ def modify_group(group, params, immediate=False):
                 if not param.is_modifiable:
                     raise NotModifiableError('Parameter %s is not modifiable.' % key)
 
-                changed[key] = {'old': param.value, 'new': new_value}
+                changed[key] = {'old': old_value, 'new': new_value}
 
                 set_parameter(param, new_value, immediate)